This invention relates to a nonlinear optical thin film having a large change of a refractive index and an optical recording medium using the nonlinear optical thin film.
As a so-called “computerized society” using optical communication has made a remarkable progress in recent years, it has become more and more necessary to build up a communication system capable of communicating large quantities of information at a higher speed. A wavelength multiplex communication system (WDM) has been expanded at present and a communication speed of a network has rapidly become higher and higher. Optical devices indispensable for expanding such a large-capacity high-speed optical communication include an optical recording medium for storing large capacity optical information and an optical switch capable of switching communication light at a high speed.
DVD (Digital Versatile Disk) having a 4.7 GB capacity on each surface has been put into practical application as the optical recording medium. This disk has now been wide spread as a medium for handling large-capacity dynamic images such as a video tape besides a computer use. DVD has been practically used as a rewritable memory besides ROM (Read-Only Memory: DVD-ROM) in which information is directly written to a substrate.
Research and development has been made to attain high recording density in such an optical recording medium. The use of a laser beam having a shorter wavelength (approx. 650 nm) than a laser beam used for CD (780 nm) has been proposed as means for attaining higher recording density in information recording.
To handle large-capacity information such as computer graphics, however, the recording density must yet be improved by 1.5 to 2 times. Therefore, development of DVD that utilizes blue-semiconductor laser using a shorter wavelength (405 nm) has been developed.
Another possible technology for attaining higher recording density is a super-resolution film. This super-resolution film is formed on an upper or lower surface of a recording medium, and high recording density can be achieved when a beam spot of incident light transmitting through, or reflected by, this film is reduced.
One of the mechanisms of the super-resolution effect is an absorption-saturation phenomenon. This is a phenomenon utilizing nonlinear optical characteristics such that the super-resolution film allows transmission of light having intensity above its absorption saturation limit but absorbs light having intensity below this level. Spatial intensity of a laser beam used for writing and reading information has a Gaussian distribution. Therefore, when the laser beam transmits the super-resolution film, light is absorbed at the skirts having low intensity but transmits the super-resolution film at the center where intensity is high. In consequence, the beam diameter after transmission can be reduced.
The super-resolution film known at present is a phthalocyanine organic film and a chalcogenide complex described in JP-A-8-96412 laid-open on Apr. 12, 1996. A use of a thermochromic material described in JP-A-6-162564 laid-open on Jun. 10, 1994 and a photochromic material described in JP-A-6-267078 laid-open on Sep. 22, 1994 for the super-resolution film has also been tried.
In the super-resolution film described in JP-A-8-96412, energy density of the beam locally reaches an extremely high level when a read or write operation is made. For this reason, the film is likely to gradually get deteriorated in the course of repetition of the write and read operations. It is therefore not easy to ensure a sufficient number of times of repetition of write/read operations under a severe environment of use as in RAM of computers, and so forth. Since chalcogenide is chemically unstable, it is difficult to acquire a sufficient guarantee term.
In the field of optical switches, on the other hand, attempts have also been made to improve the operation speed. Switching of optical information according to the prior art technologies requires opto-electric conversion for once converting optical information to electrical information at a relay point. To solve the problems such as the increase of consumed power resulting from the increase of the number of relay points and the high switching speed, however, switching is preferably conducted by light. Therefore, mechanical type optical switches, planar photo-waveguide type optical switches, mirror type optical switches and bubble type switches have been examined (Nikkei Electronics, No. 788, Jan. 29, 2001, pp.160-167).
The optical switches described above have a switching time of about several milliseconds and cannot readily cope with higher data capacity and higher operation speed of optical communication that are expected in future. In the switches described above, electrical driving force for switching is great and energy consumption yet remains great. Therefore, JP-A-11-337988 laid-open on Dec. 10, 1999 proposes a full optical switch using a nonlinear optical material and having high response by light.